In current closed loop fuel control systems for engines, the ECM (electronic control module) of an engine such as an industrial reciprocating engine generates a fuel demand signal based on various engine parameters. This fuel demand signal is compared to a measured flow rate and a valve is actuated to make any adjustment needed. Current closed loop flow control valves have problems in accurately measuring fuel flow rate that may limit their performance. The Tecjet manufactured by Woodward Governor of Fort Lauderdale, Fla. attempts to measure the flow across a butterfly valve. This method requires additional variables for calculating flow which result in increased errors. One of these variables is the area of the butterfly valve which is not linear with rotation. The area is calculated based on the measured rotation of the butterfly. Any error in the position measurement will result in an error in the flow calculation.
Determining an accurate discharge coefficient is another problem. A butterfly valve has a discharge coefficient that varies with both position and pressure. An accurate flow measurement would require a complex map of pressures, position, and corresponding discharge coefficients. In some applications, the flow across the metering valve transitions from sonic to subsonic. This requires a separate flow equation for each condition. The valve must now try to detect when this transition occurs and switch to the appropriate calculation. This introduces added complexity and potential error.
The XVG valve manufactured by Precision Engine Controls Corp. of San Diego, Calif. is similar to the Woodward Tecjet but it uses a flow metering tube instead of a butterfly valve. It suffers from the same problems associated with trying to measure flow across a variable area.
The AGV10 and AGV50 valves manufactured by Continental Controls Corporation of San Diego, Calif. also include flow measurement with closed loop flow control and use restrictive flow measurement devices with fixed areas. These valves were designed specifically for high pressure gas turbines and use a much more complex balanced poppet design for metering fuel. They also have the flow meter located downstream of the metering poppet in order to take advantage of the rising outlet pressure created by the compressor of the gas turbine. This makes an accurate flow measurement more difficult due to a non-uniform flow profile and turbulence created by the metering poppet.